1. Field of the Invention
The present invention relates to a method of assembling, into a piston ring groove of a piston, a set oil ring which comprises a spacer ring and a pair of side rail rings to be placed on both sides of the spacer ring.
2. Description of Related Art
In assembling a set oil ring, there has conventionally been employed the following procedure. Namely, a spacer ring is first assembled into a piston ring groove. Then, each side rail ring is sequentially assembled into each groove portion which is formed between the spacer ring and upper and lower groove wall surfaces, respectively, of the piston ring groove.
The assembling of the spacer ring and the side rail rings is carried out in the following manner as disclosed in Japanese Published Examined Patent Application No. 2020/1983, Japanese Published Examined Patent Application No. 19376/1986, Japanese Published Examined Utility Model Registration Application No. 16020/1986 or the like. Namely, a guide member which can be externally fitted onto an external surface of a piston is positioned relative to the piston, and then the rings which are mounted on the periphery of the guide member are pushed out of a front end of the guide member. As the guide member, there are prepared three kinds, i.e., one for the spacer ring, one for the side rail ring on the upper side and one for the side rail ring on the lower side. In each of the assembly stations, which are different from each other, the corresponding guide member is used to thereby separately assemble the spacer ring, the side rail ring on the upper side and the side rail ring on the lower side. This method has, however, a disadvantage in that the length of the assembly line becomes long and the equipment cost becomes high.
In order to solve this disadvantage, the applicants of the present application have earlier proposed the following method and apparatus in Japanese Patent Application No. 328238/1992 (corresponding to U.S. Pat. No. 5,303,465). Namely, for the purpose of definition, out of the groove wall surfaces inside a piston ring groove, the one on the side to come first in the assembling direction of the guide member relative to the piston (i.e., on the upper side) is called, "the first groove wall surface", and the one ahead, or in front, of the first groove wall surface (i.e., on the lower side) is called, "the second groove wall surface". A pair of side rail rings are mounted on a periphery of the guide member which can be fitted onto the external surface of the piston, the side rail rings are sequentially pushed out of the front end of the guide member in a condition in which the guide member is fitted onto the external surface of the piston. In this manner, the side rail rings are sequentially assembled into the first groove portion to be formed between the spacer ring that has been assembled into the piston ring groove and the first groove wall surface, and into the second groove portion to be formed between the above-described spacer ring and the second groove wall surface. The pair of side rail rings are thus assembled in a single assembly station by using a single guide member.
In this method and apparatus, the following arrangement have been made. Namely, the guide member is positioned relative to the piston such that the front end of the guide member is on substantially the same level as the first groove wall surface of the piston ring groove. The side rail ring that is positioned on the side of the front end of the guide member is first pushed out of the front end thereof in a condition in which a restricting member is oppositely disposed while leaving, in front of the guide member, a clearance or a space which faces or corresponds to the first groove portion. The side rail ring on the front side is thus assembled into the first groove portion through the clearance. Then, in a condition in which the restricting member is retracted, the side rail ring on the rear (i.e., upper) side is assembled into the second groove portion by utilizing the undulating phenomenon thereof.
By the way, the spacer ring is able to produce only a small self-maintaining force (i.e., the force to hold it fast in its position) relative to the piston ring groove. As a result, when the side rail rings are assembled into the groove portions, the spacer ring may move by the dragging or pulling by the side rail rings and consequently split end portions of the spacer ring sometimes get overlapped with each other.
In the method of the above-described proposal by the same applicants, when the side rail ring on the front side is assembled, the overlapping of the split end portions may be prevented by restricting the spacer ring with the restricting member. When the side rail ring on the rear side is assembled, however, the spacer ring can no longer be restricted. Therefore, it so happens that the split end portions of the spacer ring sometimes get overlapped.
In view of the above-described point, the present invention has an object of providing a method of assembling in which, like in the above-described method of the above-described proposal by the same applicants, the side rail rings can be assembled in a single assembly station by using a single guide member and, at the same time, in which the overlapping of the split end portions of the spacer ring can surely be prevented.